1. Field of the Invention
The present invention relates to a semiconductor device of MIS (metal insulator semiconductor) type using a :field effect and a production method thereof. More specifically, it relates to a semiconductor device in which the increase of parasitic capacitance is restrained while restraining punch-through and other effects generated by scale reduction, and a production method thereof.
2. Description of the Background Art
In order to provide a CMOS (complementary metal oxide semiconductor) having a higher performance, improvement of intrinsic performance by scale reduction is needed. Disadvantages accompanying the scale reduction include punch-through, the short channel effect, and other effects. In order to restrain punch-through and the short channel effect, a method is proposed in which the source/drain regions of first conductivity type are covered with a second conductivity type impurity region so that the source/drain regions may not directly face the channel region (Y. Okumura et. al., IEEE Trans. Electron Devices, vol. 39, No. 11, p. 9541 (1992)).
Referring to FIG. 28, in the semiconductor device disclosed in the aforesaid document by Okumura et al., a channel region 106 is disposed in a p-well 102 in a p-type silicon substrate 101. An extension region 105 extending from the source/drain regions 104 is covered with a p-type impurity region 103 so that the extension region 105 may not directly face the channel region 106. In the following descriptions, the p-type impurity region 103 will be referred to as a pocket injection region; however, in the aforesaid document, the p-type impurity region 103 is referred to as NUDC (non-uniformly doped channel) layer. A gate electrode 108 is formed on the channel region 106 with an intervening gate dielectric film 107.
If the above-mentioned pocket injection region 103 is not present, a junction is formed between the source/drain regions 104 of the n-type impurity region and the p-type well 102, 50 that when a reverse bias voltage is applied, a depletion layer extends to the p-well side, having a low impurity concentration. In other words, as the reverse bias voltage increases, the depletion layer extends towards the aforesaid channel region 106 and, depending on the degree of scale reduction of the channel length, punch-through occurs easily. If the p-type impurity concentration of the pocket injection region 103 is increased, the extension of the depletion layer towards the pocket injection region side is restrained. Therefore, by increasing the p-type impurity concentration of the pocket injection region 103, the extension of the depletion layer, which is generated in the junction formed between the source/drain regions 104 of the n-type impurity region and the p-type well 102, to the channel region 106 side can be prevented. This restrain t of the extension of the depletion layer acts effectively as a restraint of punch-through and a restraint of the short channel effect. As a result of this, scale reduction of semiconductor devices such as CMOS can be achieved without raising problems such as punch-through.
However, as shown in FIG. 28, the pocket injection region into which an impurity has been injected, i.e., ion implanted, is in contact with a bottom surface 109 of the source/drain regions over a wide range. A junction capacitance is generated at the junction of the p-type pocket injection region and the n-type source/drain regions. This junction capacitance is proportional to the area of the junction and increases as the junction becomes wider. Further, if the p-type impurity concentration of the pocket injection region is increased, the junction capacitance increases due to the decrease in the width of the depletion layer and due to the impurity concentration itself. If the junction capacitance increases, the operational characteristics of the semiconductor device deteriorate and the switching speed decreases. For this reason, there are cases in which the impurity concentration of the pocket injection region cannot be increased to a suitable level in an attempt to restrain punch-through and other effects. Scale reduction of the semiconductor devices is a perpetual goal to be achieved, and there is a need to solve these obstacles to scale reduction.
Thus, an object of the present invention is to provide a semiconductor device that makes it possible to restrain the increase of the junction capacitance and others while preventing the punch-through and others accompanying the scale reduction, and a production method thereof.
A semiconductor device of the present invention includes source and drain regions of first conductivity type disposed to sandwich a channel region located in a surface part of a semiconductor layer, and a pair of pocket injection regions of second conductivity type that cover only a neighborhood of respective side surface parts of the source and drain regions on the channel region side and form a junction only between the neighborhood of the side surface parts and the pocket injection regions.
In the above construction, the pocket injection regions and the source/drain regions form a junction at the aforesaid side surface to be in an electrically conducted state, and the pocket injection regions having a raised impurity concentration of second conductivity type restrain the extension of the depletion layer from the aforesaid junction to the pocket injection regions. For this reason, the punch-through, the short channel effect, and others accompanying the scale reduction of semiconductor devices can be restrained. Further, as described above, since the junction is limited to the side surface of the source/drain regions, the area of the junction will be extremely small as compared with the conventional semiconductor devices. Therefore, the increase of the junction capacitance, which increases in proportion to the area of the junction, is restrained, thereby preventing the deterioration of the operation characteristics and the decrease of the switching speed of the semiconductor device. As a result of this, the increase of the junction capacitance parasitic to the junction between the pocket injection regions and the source/drain regions can be restrained while preventing the punch-through, the short channel effect, and others accompanying the scale reduction of semiconductor devices.
In the aforesaid semiconductor device of the present invention, the source and drain regions include source extension and drain extension regions of first conductivity type that extend towards the channel region, and each of the pocket injection regions covers a side surface and a bottom surface of the source extension and drain extension regions on the channel region side to form a junction.
If the extension regions are formed in the source/drain regions, the pocket injection regions cover, from the lower side, the whole of the extension regions from the channel region side to the bottom surface to form a junction so that the surface of the extension regions may not be exposed to the channel region. Therefore, the extension of the depletion layer to the pocket injection region can be prevented by increasing the second conductivity type impurity concentration in the pocket injection regions. Since the extension regions are formed in an extremely limited range as compared with the source/drain regions, the area of the junction does not increase even if the whole of the extension regions that are exposed to the channel region is covered. Therefore, the increase of the junction capacitance is restrained, thereby preventing the deterioration of the operation characteristics and the decrease of the switching speed of the semiconductor device. Therefore, the increase of the junction capacitance can be restrained while preventing the disadvantages such as the punch-through accompanying the scale reduction of semiconductor devices. Here, if the whole of the side surface of the aforesaid source/drain regions is an extended part of the extensions, the pocket injection regions need not form a junction with the source/drain regions. Also, if the pocket injection regions form a junction with the source/drain regions, the junction is formed to be limited to the side surface of the source/drain regions under the aforesaid extension regions and to the bottom surface part of the source/drain regions near the corner on the lower side. Here, the terms xe2x80x9clower sidexe2x80x9d and xe2x80x9clowerxe2x80x9d refer to the side opposite to the gate electrode as viewed from the channel region, and the terms xe2x80x9cupper sidexe2x80x9d and xe2x80x9cupperxe2x80x9d refer to the side towards the gate electrode as viewed from the channel region.
In the semiconductor device of the present invention, the junction between the source and drain regions and the pocket injection regions is located successively to the junction between the source extension and drain extension regions and the pocket injection regions.
The extensions are formed at the same height level as the channel region, and the pocket injection regions are formed to cover, from the lower side, the bottom surface of the extension regions. Therefore, the pocket injection regions cover from the side surface to the bottom surface of the extension regions and form a junction at that part, and further successively cover the remaining side surface part of the source/drain regions to form a junction.
In the semiconductor device of the present invention, the junction between the extension regions and the pocket injection regions is located under the side wall spacers.
The pocket injection regions can be formed by injecting an impurity with a good precision and in a self-aligned manner into the region under the part where the side wall spacers are removed, using as a mask the gate electrode and the part which forms the upper region of the source/drain regions and which is resistant against the movement of the impurity. As a result of this, the extension of the depletion layer can be prevented and further the increase of the junction capacitance can be restrained by forming the pocket regions having an increased impurity concentration to be limited to the needed part.
The semiconductor device of the present invention further includes semiconductor compound layers disposed on the source and drain regions and respectively at positions elevated above the channel region.
By disposing a semiconductor layer or a semiconductor compound layer, which is resistant against the injection or thermal diffusion of the impurity, on the upper part of the source/drain regions as described above, the pocket injection regions are prevented from being formed at a deep position beyond the source/drain regions. Therefore, the junction formed between the pocket injection regions and the source/drain regions is not formed on the bottom surface of the source/drain regions, but is formed only on the side surface part thereof, so that the area of the junction will be extremely small as compared with the conventional case. Furthermore, an impurity of the conductivity type opposite to the source/drain regions is introduced into the lower region of the source/drain regions in forming the aforesaid pocket injection regions, and this impurity cancels the impurity concentration of first conductivity type. For this reason, the impurity concentration of the lower region of the source/drain regions will be low. Therefore, not only the decrease of the area of the junction but also the decrease of the impurity concentration contribute to the reduction of the junction leakage current and the junction capacitance.
In the semiconductor device of the present invention, the semiconductor compound layers disposed at the elevated positions are made of metal silicide.
By disposing an elevated silicon or silicon compound layer which is resistant against the injection or thermal diffusion of the aforesaid impurity, an impurity of second conductivity type can be injected, using that part and the gate electrode as a mask, into a lower region therebetween. For this reason, the pocket injection regions can be formed with a good precision and in a self-aligned manner. Further, even if the source/drain regions are formed at a shallow position in order to prevent the punch-through or the short channel effect, it does not raise a problem, as shown below. Namely, the injected impurity region which is formed in injecting an impurity into the pocket injection regions and which is successive to the pocket injection regions, will not be formed at a deep position beyond the source/drain regions. Therefore, the junction formed between the pocket injection regions and the source/drain regions is not formed on the bottom surface of the source/drain regions, but is formed only on the side surface part thereof, so that the area of the junction will be extremely small as compared with the conventional case. Further, the injected impurity region successive to the aforesaid pocket injection regions allows an impurity of conductivity type opposite to the source/drain regions to be distributed in the source/drain regions. This impurity having the opposite conductivity type cancels the impurity in the lower part of the source/drain regions, so that the impurity concentration of this part of the source/drain regions will be low. Therefore, not only the decrease of the area of the junction but also the decrease of the impurity concentration contribute to the reduction of the junction capacitance or the junction leakage current.
The semiconductor device of the present invention further includes metal films deposited on the source and drain regions and respectively at positions elevated above the channel region.
The aforesaid metal films will be resistant against the injection or thermal diffusion of the impurity in the same manner as the aforesaid elevated semiconductor layer or semiconductor oxide film, whereby the pocket injection regions are prevented from being formed on the bottom surface side of the source/drain regions. Therefore, the short channel effect and others are prevented while restraining the increase of the parasitic capacitance, and the resistance of the source/drain electrode can be reduced without forming a metal silicide layer on the source/drain electrode.
In the semiconductor device of the present invention, an impurity element heavier than an element constituting the semiconductor is injected into upper regions of the source and drain regions.
The region where the aforesaid heavy impurity has been introduced acts as being resistant to the ion injection or thermal diffusion of the impurity. This prevents the pocket injection regions from being formed on the bottom surface side of the source/drain regions and also cancels and lowers the impurity concentration in a lower layer region of the source/drain regions without physically providing an elevated structure. Therefore, the junction capacitance, the junction leakage current, and others can be prevented while restraining the punch-through and the short channel effect with the use of a compact structure. The aforesaid heavy impurity is preferably an impurity of first conductivity type.
A method of producing a semiconductor device of the present invention includes the steps of forming a gate electrode on a channel region formed from a surface of a semiconductor layer to a predetermined depth, and forming a pair of side wall spacers on both sides of the gate electrode. Further, the production method includes the steps of forming semiconductor layers, each being elevated above the channel region, on regions which is outside the pair of side wall spacers and in which source and drain regions of first conductivity type are to be formed, and removing the pair of side wall spacers. Further, the production method includes the step of forming a pair of pocket injection regions of second conductivity type by introducing, after the side wall spacers are removed, an impurity of second conductivity type below a region where the side wall spacers were formed, the pair of pocket injection regions respectively covering only a neighborhood of respective side surface parts, on the channel region side, of the region where the source and drain regions are to be formed, so as to form a junction only between the neighborhood of the side surface parts and the pocket injection region.
By the aforesaid construction, the pocket injection regions can be formed with a good precision in the lower region between the gate electrode and the elevated semiconductor layers by means of an ion injection method, a method using solid phase diffusion from BSG or PSG containing an impurity, or the like method. Since the elevated semiconductor layers are resistant against the injection or diffusion of the impurity, the impurity does not penetrate deep beyond the region where the source/drain regions are to be formed. For this reason, the injected impurity region successive to the pocket injection regions is not formed on the bottom surface side of the source/drain regions. Therefore, a junction is not formed on the bottom surface of the source/drain regions, and the junction is limited to the side part of the source/drain regions. Also, the injected impurity region which is distributed in the source/drain regions and which is successive to the pocket injection regions cancels the impurity-concentration of the source/drain regions, and lowers the concentration thereof. As a result of this, the junction capacitance and the junction leakage current can be restrained while restraining the punch-through and the short channel effect accompanying the scale reduction. Here, xe2x80x9cthe region where the source/drain regions are to be formedxe2x80x9d refers to the source/drain regions before the impurity is introduced; however, it may be simply referred to as xe2x80x9csource/drain regionsxe2x80x9d if confusion does not occur.
In the method of producing a semiconductor device of the present invention, an impurity of second conductivity type is injected into a region below the region where the side wall spacers were formed, using the gate electrode and the elevated semiconductor layers as a mask in forming the pocket injection regions.
If the ion injection method or the like is used in the aforesaid production method, the pocket injection regions can be formed in a self-aligned manner in a predetermined region using, as a mask, the gate electrode and the part which is resistant against the movement of the impurity. Therefore, the pocket injection regions can be formed with certainty in the region where it is necessary to prevent the depletion layer from being extended to the channel region side. As a result of this, the junction is not formed in an unnecessarily large amount, whereby the increase of the junction capacitance can be prevented while preventing the punch-through and others.
The method of producing a semiconductor device of the present invention further includes a step of respectively forming source extension and drain extension regions of first conductivity type that extend from the region where the source and drain regions are to be formed, by introducing an impurity of first conductivity type into a region which is in contact with a surface of the pocket injection regions on a side of the region where the source and drain regions are to be formed, successively after the pocket injection regions are formed.
By the aforesaid production method, the pocket injection regions and the source extension and drain extension regions can be formed in the same step, thereby preventing the large increase of the number of steps. Needless to say, in the above method, the ion injection method of bombarding with ions or the method of using solid phase diffusion from BSG or PSG containing an impurity, and other methods are to be used.
The method of producing a semiconductor device of the present invention further includes, after the step of respectively forming the source extension and drain extension regions, a step of forming side wall spacers again in the region where the side wall spacers were formed, thereafter a step of introducing an impurity of first conductivity type into the region where the source and drain regions are to be formed, so as to form the source and drain regions, and a step of turning the elevated semiconductor layers into metal silicide.
By the above construction, the source/drain regions can be formed using the reproduced side wall spacers as a mask. Typically, a heating treatment for activating the impurity is carried out in forming the source/drain regions. At this time, the impurity of first conductivity type is injected so that the region successive to the pocket injection regions that have been previously formed to be elevated will all be the source/drain regions of first conductivity type. As a result of this, the increase of the junction capacitance can be restrained while restraining the short channel effect accompanying the scale reduction of the semiconductor devices.
The method of producing a semiconductor device of the present invention further includes, between the step of forming the elevated semiconductor layers and the step of removing the side wall spacers, a step of introducing an impurity of first conductivity type through the elevated semiconductor layers into a lower region to form the source and drain regions.
In the above production method, the step of forming the pocket injection regions and the source extension and drain extension regions is carried out after the source/drain regions are formed. As described above, for forming the source/drain regions, a heating treatment for activating the impurity is carried out after the introduction of the impurity. As a result of this, the heat history received by the impurity of the pocket injection regions and the source extension and drain extension regions can be reduced, and a sharp rise and fall of the impurity profile can be formed in the aforementioned region. As a result of this, the extension of the depletion layer can be effectively prevented to restrain the punch-through and the short channel effect. Also, since the junction is not formed at unnecessary locations, the parasitic junction capacitance can be restrained.
The method of producing a semiconductor device of the present invention further includes, after the steps of forming the source and drain regions, removing the side wall spacers, and respectively forming the pocket injection regions and the source extension and drain extension regions, a step of forming side wall spacers again in a region where the side wall spacers were formed and a step of turning an upper part of the elevated semiconductor layers into metal silicide.
By the above construction, since the heating treatment after the introduction of the impurity into the source/drain regions is usually carried out already before the pocket region injection, the heat history to the impurity in the pocket injection regions and the source extension and drain extension regions can be reduced. As a result of this, a sharp profile of the impurity concentration in these regions can be formed.
The method of producing a semiconductor device of the present invention further includes, after the step of forming the source and drain regions and before the step of removing the side wall spacers, a step of turning the elevated semiconductor layers into metal silicide.
By the above construction, since the heating treatment after the introduction of the impurity into the source/drain regions is usually carried out already before the pocket region injection, the heat history to the impurity in the pocket injection regions and the source extension and drain extension regions can be reduced. As a result of this, a sharp profile of the impurity concentration in these regions can be formed. Also, since the process can be carried out without forming an outer layer of the side wall spacers, the number of steps for forming the second side wall spacers can be reduced.
In the method of producing a semiconductor device of the present invention, the step of forming the side wall spacers preferably includes a step of forming the side wall spacers so as to form a two-layer structure having a lower layer of silicon oxide film and an upper layer of silicon nitride film. Also, the step of removing the side wall spacers preferably includes a step of removing the outer layer of silicon nitride film and leaving the inner layer of silicon oxide film.
Since the materials constituting the above-mentioned two-layer structure can be etched with good selectivity to each other, the outer layer can be removed with good precision, for example, for forming the pocket injection regions. Therefore, the gate electrode and others are not exposed to an atmosphere during the production, so that the aforesaid semiconductor device can be produced to have a high reliability.
Also, if it is preferable to form the pocket injection regions and others by ion injection with a low energy, the step of removing the side wall spacers includes a step of removing both the outer layer of silicon nitride film and the inner layer of silicon oxide film.
Since the silicon oxide film on the surface of the semiconductor layers above the pocket injection regions or the extension regions is removed, the ion injection can be carried out with a low energy. Therefore, the pocket injection regions and the extension regions having a sharp impurity concentration distribution can be formed. Here, it goes without saying that the above-mentioned side wall spacers refer not only to the side wall spacers formed first but also the second side wall spacers formed again.
In the method of producing a semiconductor device of the present invention, in the step of forming the pocket injection regions, an impurity of second conductivity type is introduced by ion injection method, and its injection angle xcex8 from the vertical direction is set to be not larger than tanxe2x88x921(Ls/Te). Here, Ls=width of the removed side walls, and Te=height by which the source and drain regions are elevated from the channel region.
By the above-mentioned production method using the ion injection method, the pocket injection regions can be formed to be extended towards a desired direction from the region of the part where the aforesaid side wall spacers have been removed. As a result of this, the injection angle xcex8 can be changed and optimized in accordance with the change of design or the variation at the time of production.
In the method of producing a semiconductor device of the present invention, it is preferable that a facet surface is formed at an upper part of the elevated semiconductor film on the gate side, and the facet surface is disposed so that the distance between the gate and the facet surface increases in an upward direction.
By this construction, the ion injection angle can be increased even with the same elevated height Te and with the same width Ls of the removed side walls. Therefore, the pocket injection regions can be easily formed at an intended position. Thus, the junctions of the pocket injection regions with the extension regions and the source/drain regions can be accurately limited.
In the method of producing a semiconductor device of the present invention, after the side wall spacers are removed, a deposition layer containing an impurity of second conductivity type is formed and then a heat treatment is carried out so that the impurity of second conductivity type is introduced by thermal diffusion from the deposition layer containing the impurity of second conductivity type, thereby to form the pocket injection regions.
By forming the pocket injection regions by thermal diffusion from the aforesaid dielectric film, the impurity distribution of the regions can be made to have a sharp profile.
In the method of producing a semiconductor device of the present invention, instead of the step of forming the semiconductor layers to be elevated above the channel region, either one of an oxide film or a nitride film of the semiconductor is formed to be elevated above the channel region by either one of a thermal oxidation method and a thermal nitriding method.
The aforesaid semiconductor-oxide film or nitride film will be resistant in the ion injection or thermal diffusion of the impurity. Therefore, in forming the pocket injection regions, the part successive to the pocket injection regions does not go beyond the source/drain regions to be formed on the bottom surface side thereof. For this reason, the pocket injection regions form a junction only with the neighborhood of the side surface part of the source/drain regions on the channel region side. As a result of this, the increase of the junction capacitance can be restrained while preventing the short channel effect. With respect to the aforesaid oxide film and nitride film, the aforesaid elevated oxide layer or the elevated nitride layer can be formed using an apparatus that has been heretofore put to use, so that the rise in the production costs can be restrained.
In the method of producing a semiconductor device of the present invention, instead of the step of forming the semiconductor layers to be elevated above the channel region, a metal layer is formed to be elevated above the channel region.
The aforesaid metal film performs the same function as the aforesaid elevated semiconductor layer, semiconductor oxide film, or semiconductor nitride film. In other words, the aforesaid metal film will be resistant in the ion injection or thermal diffusion of the impurity. Therefore, the pocket injection regions are prevented from going beyond the source/drain regions to be formed on the bottom surface side thereof. Therefore, the short channel effect and others can be prevented while restraining the increase of the parasitic resistance, and the junction of the source/drain regions can be reduced to restrain the increase of the junction capacitance. Further, by using the aforesaid metal film, the resistance of the source/drain electrode can be reduced without forming a metal silicide layer on the source/drain electrode.
In the method of producing a semiconductor device of the present invention, instead of the step of forming the semiconductor layers to be elevated above the channel region, an impurity element heavier than an element constituting the semiconductor is introduced into an upper part of the source and drain regions.
The region where the aforesaid heavy impurity has been introduced will be resistant in the ion injection or thermal diffusion of the impurity, like the aforesaid elevated semiconductor layer, semiconductor oxide film, semiconductor nitride film, or metal film. Therefore, the pocket injection regions are prevented from going beyond the source/drain regions to be formed on the bottom surface side thereof. Therefore, the short channel effect and others can be prevented while restraining the increase of the parasitic resistance, and the junction of the source/drain regions can be reduced to restrain the increase of the junction capacitance. Also, the region which will be resistant to the movement of the impurity can be formed without physically providing an elevated structure. Therefore, the increase of the junction capacitance can be prevented while producing a MOSFET structure having an ordinary structure with the use of an ordinary MOSFET producing apparatus, and while preventing the punch-through and the short channel effect accompanying the scale reduction. Here, the aforesaid heavy impurity element is preferably an impurity of first conductivity type.
The foregoing and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.